Narrow band electric discharge gas laser having improved beam direction stability

ABSTRACT

An electric discharge, narrow band gas laser with improvements in wavelength stability. Improvements result from reduced laser beam directional fluctuations or fast correction of those fluctuations. Applicant has discovered, using an extremely sensitive knife edge optical technique, that gas discharge laser windows in a trapezoidal configuration were causing slight wavelength perturbations when laser gas density varied during laser operation. The optical technique involves using test laser beam directed through the discharge region of the gas discharge laser, blocking a portion of the beam with a knife edge and measuring the non-blocked portion of the beam to monitor beam deflection. With this technique, Applicant can measure beams deflection with an accuracy of about 0.3 microradians and with a time response of about 1 microsecond. An improvement in wavelength stability is achieved by orienting the laser chamber windows parallel to each other at a selected angle between 40° and 70° with the laser beam direction. The change eliminates wavelength fluctuations caused by laser beam direction fluctuation caused pressure fluctuations and the prism effect resulting from windows mounted in a prior art trapezoidal configuration. Beam directional fluctuations can also be measured during laser operation using the knife edge technique or similar fast response techniques such as a quadrant detector and the measured values can be used in a feedback arrangement along with fast wavelength control unit to compensate for changes in beam direction. In addition techniques for reducing the causes of beam direction fluctuations are disclosed. These include techniques for minimizing the effects of laser discharge caused pressure waves.

This invention is a continuation-in-part of Ser. No. 09/490,835 filedJan. 25, 2000. This invention relates to gas discharge lasers andespecially to narrow band, high pulse rate electric discharge gas laserswith rapidly circulating laser gas and to methods of testing and ofstabilizing laser beam direction in such lasers.

BACKGROUND OF THE INVENTION

Gas lasers typically contain a laser gas contained in a laser chamber.Normally two windows are provided for a laser beam to pass into and outof the chamber. These windows may be positioned normal to the laser axisor they may be positioned at various angles with respect to the beamaxis. If the mirrors are normal to the laser beam axis the beam willsuffer about 4% reflection loss at each window-gas interface andreflection from windows in the normal position may be detrimental to thelaser gain and energy stability. By tilting the windows, even slightly,reflection caused problems can be reduced. By tilting the windows to theBrewster's angle (typically about 57°), the windows will have virtually100% transmission for light whose electric component is parallel to theplane of incidence (defined by the beam axis and a normal to the windowsurface at the intersection of the beam axis and the window surface) seeFIG. 1 at 7A and 7B. At angles smaller or larger than the Brewster'sangle transmission of the parallel component is reduced by an amountdependent on the amount of deviation from the Brewster's angle.

As shown in FIG. 1, having both windows tilted at angles parallel toeach other produces an offset due to the lateral displacement of thebeam in the same direction in both windows. This complicates thealignment of the front and rear laser optics especially for large gasdischarge lasers laser systems which typically are assembled fromseparate modules which are mounted separately on a laser frame.Therefore, in prior art gas discharge lasers with tilted windows, thewindows are tilted in opposite directions as shown in FIG. 2 at 7C and7D. This produces a “trapezoidal” shaped beam path. This arrangement ofthe mirrors also makes inspection of the windows for damage relativelyeasy since both windows can be viewed from the front of the chamberwhere doors to the laser cabinet are typically located. FIG. 2 describesa prior art excimer laser system used as the light source for integratedcircuit fabrication. In this system, the laser chamber, the rear opticand the front optic are each separate modules mounted separately on theframe of the laser system.

The rear optic is a line narrowing module (typically called a linenarrowing package or LNP) 2 and the front optic is an output couplermodule 4. The laser chamber 6 can be removed from the front of the lasersystem and replaced or realigned without disturbing either the front orthe rear optics and without significantly affecting the opticalalignment. This is because the offset caused by one of the windows iscancelled by the other window. The LNP includes a three-prism beamexpander 8, a tuning mirror 10 and a grating 12 positioned in a Littrowconfiguration.

FIG. 3 is a cross-sectional sketch of laser chamber 6 shown in FIG. 2.The chamber in addition to the laser gas contains elongated cathode 36A,elongated anode 36B, preionizer tube 46, insulator 42, anode support bar35, heat exchanger 40 and tangential fan 38 for circulating the lasergas fast enough to clear discharge region 34 between successive pulses.For a 2,000 Hz pulse rate, this requires a gas velocity between theelectrodes of about 30 m/s (about 67 miles/hour).

In this prior art laser system which typically operates at pulserepetition rates of 1000 to 2000 Hz, both the pulse energy and thewavelength are controlled with feedback control systems in which eachlaser pulse is monitored by power and wave meter 14 and the measuredvalues are used by controller 16 to control the energy and wavelength ofsubsequent pulses based on measured values of pulse power andwavelength. Pulse energy control is achieved by controlling the chargeon a charging capacitor bank in pulse power system 18 and thewavelengths of subsequent pulses are controlled by automatic adjustmentof tuning mirror 10 by adjusting a drive arm of drive motor 20 to pivotthe mirror. In this prior art laser system a change in the angle ofillumination on the grating of 1.0 milliradian will result in a changein the selected wavelength of about 39 pm. A change in the direction ofthe beam exiting the chamber will also change the selected wavelengthbut because of the 26× prism beam expander the effect is a factor of 26less. Therefore, a 1.0 milliradian change in the direction of the beamexiting the chamber will cause only a 1.5 pm. change in the selectedwavelength.

As is evident from FIG. 2, the laser gas in the beam path within thechamber has the shape of a trapezoid which like a prism causes a veryslight bending of the laser beam. For an ArF excimer laser withtrapezoidal 45° mirrors with a three atmosphere mixture of 96.5% neon,3.4% argon and 0.1% fluorine, the bending angle is 366 microradian ascompared to a complete vacuum in the chamber. For the ArF laser shown inFIG. 2, a bending of the laser beam of 366 microradians corresponds to achange in the selected wavelength of 0.54 pm. This change is very smallcompared to the tuning range of tuning mirror 10, so that the bending ofthe beam caused by the gas “prism” is automatically compensated for bythe wavelength feedback control. Also, a small quickly occurring changein the gas pressure during operation produces a very slight change inthe wavelength which might be too fast for correction by normal feedbackcontrol. For example, a 2% change in the pressure at the same gastemperature would produce a wavelength change of about 0.011 pm.

In the past, operational variations in wavelength for these types oflasers have typically been in the range of about ±0.3 pm., and laserspecifications on wavelength stability have been about ±0.5 pm.Therefore, in the past wavelength fluctuations (in the range of about0.011 pm.) caused by small laser gas pressure changes in the trapezoidalshaped contained laser gas has not been recognized as a problem.Furthermore, as indicated above, if the pressure change is long comparedto the wavelength feedback control cycle (which typically has been lessthan about seven milliseconds) any wavelength deviation from a targetwavelength would be quickly and automatically corrected.

A known technique for measuring changes in light direction is to focusthe beam on a spot partially blocked by a knife edge and to monitor theintensity of light not blocked. Changes in the intensity is a measure ofthe beam fluctuation.

Efforts have been made recently by Applicants and others to reducewavelength variations and specifications on wavelength stability havebecome tighter.

What is needed in laser equipment and techniques to improve wavelengthstability.

SUMMARY OF THE INVENTION

The present invention provides an electric discharge, narrow band gaslaser with improvements in wavelength stability. Improvements resultfrom reduced laser beam directional fluctuations or fast correction ofthose fluctuations. Applicant has discovered, using an extremelysensitive knife edge optical technique, that gas discharge laser windowsin a trapezoidal configuration were causing slight wavelengthperturbations when laser gas density varied during laser operation. Theoptical technique involves using test laser beam directed through thedischarge region of the gas discharge laser, blocking a portion of thebeam with a knife edge and measuring the non-blocked portion of the beamto monitor beam deflection. With this technique, Applicant can measurebeams deflection with an accuracy of about 0.3 microradians and with atime response of about 1 microsecond. An improvement in wavelengthstability is achieved by orienting the laser chamber windows parallel toeach other at a selected angle between 40° and 70° with the laser beamdirection. The change eliminates wavelength fluctuations caused by laserbeam direction fluctuation caused pressure fluctuations and the prismeffect resulting from windows mounted in a prior art trapezoidalconfiguration. Beam directional fluctuations can also be measured duringlaser operation using the knife edge technique or similar fast responsetechniques such as a quadrant detector and the measured values can beused in a feedback arrangement along with fast wavelength control unitto compensate for changes in beam direction. In addition techniques forreducing the causes of beam direction fluctuations are disclosed. Theseinclude techniques for minimizing the effects of laser discharge causedpressure waves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art Brewster's window arrangement for a simple gaslaser design.

FIG. 2 shows important features of a prior art high repetition rate,narrow-band gas discharge laser used for integrated circuit lithography.

FIG. 3 shows a cross section of the laser chamber depicted in FIG. 2.

FIG. 4 shows a test arrangement utilized by Applicant.

FIG. 5 shows some of Applicant's test data.

FIG. 6 shows a modified test arrangement to prove the benefits of thepresent invention.

FIG. 7 shows a “before” and “after” comparison demonstrating thebenefits of the present invention.

FIG. 8 shows revisions to the high repetition rate narrow band gasdischarge laser to incorporate an embodiment of the present invention.

FIGS. 9, 10A and 10B are graphs showing beam deflections monitored usinga special knife edge optical technique.

FIGS. 11A through 11D2 show laser features used to reduce adverseeffects of discharge caused pressure waves.

FIG. 12 is a drawing of an LNP with a fast response piezoelectric driverused to turn the tuning mirror.

FIGS. 13 and 14 show techniques for measuring beam deflections duringlaser operation.

FIG. 15 shows a technique for active control of wavelength fluctuationsdue to beam direction fluctuations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Knife Edge Experiments

Applicant has conducted extensive experiments to determine the causesand effects of beam bending within the laser gas of the type of electricdischarge lasers described in FIG. 2. A slightly simplified version ofhis basic experimental set up is shown in FIG. 4. The output coupler andthe LNP were removed and replaced with a transilluminating diode laser(Model No. 31-0334 by Coherent, Inc. with offices in Santa Clara,Calif.) 30, lens 31, a diode detector (Model No. DET 200 by ThorLabs,Inc. with offices in Newton, N.J.) 32 and a knife edge 34 which cutabout 50 percent off from one side of the laser beam all as shown inFIG. 3. Diode laser 30 is a small inexpensive laser producing acontinuous visible light, red beam at a wavelength of 635 nm. Anybending of the beam shows up as a change in the intensity of the lightfrom laser 30 measured by detector 32. During a set of tests, the laserwas discharged and the fan was operated at a normal speed of about 3800rpm which would be sufficient for 2000 Hz repetition rate. There was nolasing generated in the laser gas because the laser optics were not inplace. (The simplified drawing does not show a pyrex plate whichApplicant placed just upstream of lens 31 or a turning mirror justdownstream of lens 31. The pyrex plate blocked discharge generated lightand the turning mirror merely made placement of the optics moreconvenient.)

Some of Applicant's results are shown in FIG. 5. This figure presentsfive sets of beam deflection data taken at different times (but plottedtogether in FIG. 5) as a function of time just prior to and for a periodof 2 milliseconds after an electrode discharge. The laser repetitionrate was low (less than 100 Hz) so that only one pulse is represented ineach two-millisecond plot. The laser gas was held at nominal pressuresof 489 kPa, 450 kPa, 400 kPa, 350 kPa and 313 kPa, respectively, for thefive sets of data. The data for each of the five sets were plotted onthe same chart with the discharge synchronized at time zero in eachcase. The data clearly shows a linear variation of beam deflection withnominal laser gas pressure. This relationship is about 1.25 μrad/kPa forthis chamber with 45° windows mounted trapezoidally. Applicant'sexperimental beam deflection as a function of laser gas pressure agreesalmost perfectly with the corresponding value of prism beam deflectioncalculated using published index of refraction data for the laser gasmixture. Applicant then modified the laser configuration shown in FIG. 4at 7E and 7F by orienting the windows into a parallel arrangement asshown in FIG. 6 at 7G an 7H. He then repeated the test as describedabove at various nominal pressures and the result was no variation inbeam direction with nominal pressure.

Applicant's data plotted in FIG. 5 also shows enormous deflections ofthe beam during an approximately 0.25 millisecond period following eachburst. This perturbation is due primarily to very large densitygradients across the beam path which result from the discharge. With thefan speed used in this experiment, it takes about 0.25 ms for theflowing gas in this case to push the discharge disturbed laser gas outof the beam path. The increase in deflection seen at about 0.45 ms afterthe discharge (zero time) is due to reflection of the acoustic wavesfrom the chamber walls which as shown in FIG. 3 is about 9 cm fromlocation of the discharge. The shock wave travels in the laser gas about21 cm in 0.45 ms and produces density gradients in the beam path when itreturns to the discharge region which in turn bend the beam.Perturbations of the type shown at about 0.45 ms in FIG. 5 can beminimized using the technique discussed in U.S. patent application Ser.No. 09/490,835 filed on Jan. 25, 2000 by Applicant and some of hisfellow workers. (Techniques for minimizing the effects of wallreflections are shown in FIGS. 11A through 11D2 which are copied fromthat application. FIG. 11A shows a cross-section of a laser chamber withbaffles attached by screws to the chamber walls at positions 60, 62, 64and in the upper corners as shown at 66 and 68. The baffles have a crosssection which is saw-tooth shaped with varying shaped teeth as shown inFIG. 11B1 and 2 which are end views of baffle 60 and a portion of baffle60. As indicated in detail views of portions of baffle 60 (FIG. 11B1 andFIG. 11B2), the pitch of the teeth vary from 0.390 inch to 0.590 inchand the height of the teeth vary from 0.120 inch to 0.280 inch. Theteeth are aligned generally in the direction of gas flow andperpendicular to the laser beam direction and the long dimension of thedischarge region. In this preferred embodiment the baffled material is20 gage nickel plated aluminum sheet. This baffle design is veryeffective in dispersing the discharge produced pressure waves. Thisdesign reflects the waves in a great number of directions with minimumreflection in directions perpendicular to the long direction of thedischarge region. The result is that if and when acoustic energy fromany particular pulse returns to the discharge region, the energy (orpressure perturbation) of the wave is fractionalized into a very largenumber of pieces, and thus the net index of refraction gradientsproduced in the beam path are reduced.

Beam Deflections Due to Fan Caused Short Term Density Fluctuations andPrism Effect

As indicated above, the beam direction changes at the rate of 1.25μrad/kPa with pressure fluctuations in the laser gas (causing densityfluctuations) due to the prism effect caused by the trapezoidal shapedlaser gas profile. Since the nominal laser gas pressure is about 400 kPafor an ArF laser a 2% fluctuation in the gas density (corresponding to 8kPa) would result in a beam deflection of 10 μrad. Deflection of thismagnitude would change the selected wavelength by about 0.015 p.m.

Applicant suspected that the trapezoidal shaped laser gas combined withfan caused short term pressure fluctuations (in the range of about 2%)were causing fluctuations in laser beam direction which were in turncausing some of the wavelength fluctuations shown in FIG. 5. In order totest this theory, Applicant operated the laser with the fan running atnormal speed but with no discharge and measured the test beamfluctuations with the set up shown in FIG. 4. He then reversed the tiltdirection of one of the chamber windows so that the windows were alignedparallel to each other as shown in FIG. 6 and he compared beamdeflection data from the FIG. 6 configuration to similar data from theFIG. 4 configuration. The results are shown in FIG. 7. The left side ofthe graph is a plot of data taken with trapezoidal arranged windows andthe right side of the graph is a plot of data taken with parallelarranged windows. This comparison in FIG. 7 clearly confirms Applicant'ssuspicions. Although not shown, in both cases, fluctuations in beamdirection virtually disappeared when the fan was shutdown.

From the FIG. 7 comparison, Applicant has determined that thetrapezoidal shape of the contained laser gas (under the influence of fancaused density changes) is responsible for wavelength instabilities ofabout 0.012 p.m. Based on these tests, Applicant has redesigned thelaser system shown in FIG. 2 as shown in FIG. 8 at 7G and 7H toincorporate parallel chamber windows both tilted at an angle of about45° with the beam direction. The resulting improvement in laser linecenter stability of about ±0.012 pm. represents a reduction from thetypical present day stability of about ±0.3 pm. to about 0.29 pm. Whileimprovements may seem small, it is nevertheless significant, and asother causes of wavelength fluctuations are eliminated or minimized,this 0.012 pm becomes relatively more important. Also, the advantage andpositive effect of the parallel windows becomes more important as fanspeed and gas velocity increase. Such increases are expected in the nearfuture for these types of lasers to satisfy a need in the integratedcircuit fabrication business for greater productivity.

Other Improvements

As indicated in FIG. 7, although Applicant, by changing the windowdesign has made a significant improvement in these narrow band gasdischarge lasers, a lot is left to be done in terms of minimizing lasergas related beam deflections. Applicant believes that most of the beamdeflection (shown at the right side of FIG. 7) left after correcting thewindow design is due to turbulent pressure cells created in the fastflowing laser gas. Beam deflections can be further reduced by makingchanges within the chamber to make these turbulent cells smaller.Applicant suspects that substantial improvements can be made by applyingknown techniques utilized in fan designs in other industries which haveover the years been implemented mainly to reduce fan noise.

Also it is possible to speed up the feedback wavelength controldiscussed above by utilizing faster components such as piezoelectricdrivers and faster computer systems and techniques for control. FIG. 12shows an improved LNP which incorporates a piezoelectric driver 86 toprovide very fast adjustment of turning mirror 10 to permit pulse topulse feedback control of the wavelength. Large movements of the mirrorare accomplished with slower response stepper motor 20.

Knife Edge Test Equipment

Applicant's test setup shown in FIGS. 4 and 6 is an extremely valuabletechnique for measuring beam deflections in gas discharge lasers and forpermitting scientists to identify causes of rapid beam fluctuations.Applicant has used this technique to identify the extent to whichtemperature gradients in the gain region cause a bending of the laserbeam and a corresponding change in wavelength. This temperature gradientoccurs during burst mode operation such as an operating cycle where thelaser is operated in bursts of pulses of for example 300 pulses at 1000Hz each burst followed by a down time of 0.3 seconds as described inU.S. Pat. No. 6,034,978. It takes about 45 milliseconds for gas tocirculate in a laser chamber such as the one shown in FIG. 3. Therefore,about 45 milliseconds after the start of a burst of pulses, hot gasproduced by the first pulse of the burst returns to the discharge regionproducing a shift temperature (and density) gradient across the gainregion. This density gradient across the beam path causes a bending ofthe laser beam in the gain region and causes slight changes in thewavelength. This returning (first pulse disturbed) gas also containsatomic and molecular structures which vary from equilibrium and thushave an effect on the index of refraction in the beam path causing someperturbations in the beam direction. Applicant has used his knife edgetechnique to investigate these effects.

FIG. 9 shows some of the results of one of these experiments. In thiscase, a laser was operated with a setup as shown in FIG. 4 at 1000 Hzfor a period of 0.01 second (10 pulses) and data was recorded first withthe knife edge at the left of the beam then with the knife edge at theright of the beam and the recorded data was plotted superimposed asshown in FIG. 9. These plots clearly show the bending of the beam byabout 7 microradians. As indicated above this 7 microradian disturbancesin the beam direction unless corrected or compensated for would resultin a wavelength shift of about 0.0105 pm.

FIG. 10A is an expanded graph showing the first 12 milliseconds of theFIG. 9 graph and FIG. 10B is a further expanded graph showing only thefirst 1.0 millisecond of the FI. 9 graph. These graphs show the almostunbelievable sensitivity of this technique for measuring beamdeflections in gas discharge lasers. Applicant estimates that theangular resolution of this technique is in the range of 0.3microradians. The reader will note by comparing FIG. 10B and 9 that thelaser beam at about 0.00025 seconds after the pulse is bent in adirection opposite the bending direction at 0.045 seconds after thepulse indicating that the pulse disturbed gas leaving the dischargeaffects the index of refraction in a manner opposite to its effect onreentering the discharge region.

Active Feedback Control of Beam Directional Fluctuations

FIG. 13 shows a technique of measuring beam deflections during laseroperation of a line narrowed gas discharge laser. A diode laser 30produces a CW beam which is reflected of the back surface of the laseroutput coupler 80. (The back surface is coated with an anti-reflectivecoating for UV light but it nevertheless reflects about 4 percent of thediode laser beam which is in the visible range at 635 nm. The reflectedportion of the diode laser beam transverses the laser chamber and aportion of the beam reflects off the rear chamber window 82 tophotodiode detector 32A which measures the beam deflection within thelaser chamber. Most of the diode laser light will pass through rearwindow 82 substantially aligned with the gas discharge laser beam whichis a short pulse UV laser beam. This portion will be expanded in thethree prism beam expander, reflect off the tuning mirror 10 and the zeroorder reflection off grating 12 will be retroreflected off mirror 84 andreturned to window 82 where a portion will be reflected to photodiode32B which will detect the net beam deflections in both the chamber andthe LNP. This arrangement measures deflections in only the horizontaldirection. By using a quadrant detector as shown in FIG. 14, deflectionsin both horizontal and vertical directions can be monitored. Asdescribed above, the knife edge detector can monitor beam direction witha time sensitivity of 1 microsecond. The most important time segmentwill be a few microseconds just prior to the discharge. Therefore,monitoring hardware and software should be provided which will permitthe monitoring of the knife edge data during a few microsecond period,just prior to the discharge. This data will be representative of the UVbeam direction for the immediately following pulse.

FIG. 15 shows a technique for feedback control of beam direction tosubstantially eliminate fluctuations in wavelength due to fluctuationsin beam direction. In this case, the signal from photodiode 32B is usedby feedback control unit 88 to drive very fast response PZT driver 86 tocontrol the position of tuning mirror 10. Control unit 88 controlsdriver 86 to maintain the signal from photodiode detector 32B at asubstantially constant value. The result is that light reflected offgrating 12 is reflected at a substantially constant angle. Since thebeams from both the diode laser and the gas discharge laser followsubstantially the same path through the LNP, the gas discharge laserbeam must reflect off the grating at a substantially constant angle.This means that the selected wavelength will be substantially constant.This arrangement permits adjustment of tuning mirror 10 during periodswhen the gas discharge laser is idle. This permits the system to correctfor wavelength drift during idle periods when the prior art wavelengthfeedback mechanisms are not available. This arrangement also permitspre-tuning of wavelength prior to the start of discharge operation.

The reader should note that additional control could be achieved byusing the quadrant detector shown in FIG. 14 and replacing mirror 10with a mirror which can be controlled in two directions of rotation sothat both horizontal and vertical beam direction fluctuations could becorrected. Correcting for vertical beam fluctuations is not necessaryfor wavelength control but could be useful for control of pulse energyfluctuation due to beam deflections in the vertical direction. Applicantestimates based on knife edge experiments to test for vertical beamfluctuations that vertical beam fluctuations can be the cause of pulseenergy variations in the range of about 2 to 5 percent. Thesefluctuations could be substantially eliminated by a feedback controlsimilar to the one described in detail above for compensating forhorizontal fluctuations. An alternate to the use of the quadrantdetector would be to use a second knife edge detector system butpositioned to measure only vertical beam fluctuations. This could beused to control the rotation of mirror 10 about a horizontal pivot axisor an additional tuning mirror could be placed in the beam path forvertical beam deflection compensation.

Various modifications may be made to the present invention withoutaltering its scope. For example, the windows could be oriented at anglesother than about 45° and the Brewster's angle. Applicant recommends ageneral range between about 40° and 70°. Those skilled in the art willrecognize many other possible variations. Accordingly, the abovedisclosure is not intended to be limiting and the scope of the inventionshould be determined by the appended claims and their legal equivalents.

1. A narrow band electric discharge gas laser with minimized wavelengthvariations caused by fluctuations in laser gas density resulting inlaser beam directional changes comprising: A) a laser chamber, B) anelongated electrode structure enclosed within said chamber comprising anelongated anode and an elongated cathode separated by a distancedefining a discharge region in which a discharge laser beam isamplified, said discharge region defining a long dimension in a beamdirection, C) a laser gas contained in said chamber, D) a fan forcirculating said laser gas within said chamber and through saiddischarge region, E) an output coupler, F) a grating based linenarrowing module comprising a grating and a tuning means to controldirection of illumination of light from said chamber on said grating,said direction of illumination defining an illumination direction, (G) atest laser producing a test laser beam directed along a path throughsaid discharge region and into said line narrowing module and reflectingat least once off said grating, H) a fast beam deflection monitoringmeans to monitor deflection of said test laser beam; and I) a feedbackcontrol means for controlling said tuning means based on signals fromsaid beam deflection monitoring means.
 2. A laser as in claim 1 whereinsaid tuning means is a pivoting mirror.
 3. A laser as in claim 1 whereinsaid beam deflection monitoring means comprises a knife edge blocking aportion of a sample of said test laser beam and a detector formonitoring intensity of said sampled portion downstream of said knifeedge.
 4. A laser as in claim 1 wherein said beam deflection monitoringmeans comprises a quadrant detector.
 5. A laser as in claim 1 andfurther comprising a pulse energy control means for minimizing pulseenergy fluctuations caused by discharge laser beam fluctuations in avertical direction.
 6. A laser as in claim 5 wherein said tuning mirrorpivots about each of two axis and said feedback control means comprisesmeans for controlling degrees of pivot about both axis based on signalsfrom said beam deflection monitoring means.
 7. A laser as in claim 1wherein said feedback control means comprises means for pretuning saiddischarge laser prior to beginning of lasing operation.
 8. A laser as inclaim 1 wherein said feedback control means comprises means forcorrecting for wavelength drift during idle periods of said dischargelaser.
 9. A technique for measuring effects of changes in beamdeflection in a narrow band gas discharge laser defining a dischargeregion comprising the steps of: A) directing a test laser beam throughsaid discharge region; B) blocking a portion of said test laser beamdownstream of said discharge region with a knife edge; and C) monitoringa non-blocked portion of the beam to determine changes in beam directioncaused by deflections of the beam in the discharge region.
 10. Atechnique for providing active feedback control of laser beamdirectional fluctuations in a narrow band gas discharge laser systemhaving a grating based line narrowing unit and defining a gas dischargeregion and a discharge laser beam path through said discharge region andinto and out of said line narrowing unit comprising the steps of: A)directing a test laser beam through said discharge region and said linenarrowing unit along a path at least partly co-aligned with saiddischarge laser beam path; B) directing a portion of said test laserbeam to a deflection detector to produce a signal to monitor deflectionof said test laser beam; and C) using said signal in a feedback loop toprovide rapid control wavelengths of said narrow band gas dischargelaser system in order to minimize fluctuations in wavelength due to beamdirectional fluctuations, pretune the laser and/or to correct forwavelength drift.